My 550EX works in ETTL mode when directly attached to my 10d, and my Ellan II. However, once the cord is introduced into the mix I go into TTL mode (on both cameras)

Is one of the 4+1 pins specifically involved in the 'E' of 'E-TTL' ?

I agree with englishm - try a new cord. Perhaps go to a local camera store, and see if they've got one they'll let you compare against?? Since it's doing it with both cameras, and the flash works fine otherwise, this would be the first logical troubleshooting step. AFAIK, the flash and camera shouldn't notice any difference with the cord in between them....

Somewhere out on photo.net I recall seeing a pointer to the pin usage for E-TTL vs. normal TTL metering on these cameras, but I don't seem to have it handy anymore. A search there might turn up some help...